Our transcription: One of the most fundamental aspects of the Parkfield Experiment focuses on the structure of the San Andreas Fault itself. To learn more about this structure, geophysicists have set up the Vibra-seis Project. At the heart of this effort is a specially equipped truck that shakes the ground, triggering waves of seismic energy. Radiating into the earth, the seismic waves moves at different velocities through different rock types. Analysis of the velocity changes makes it possible to unravel the intricacies of the subsurface geologic structure. As the waves penetrate the Earth, they are reflected and refracted off the various rock layers, and by measuring first the direct wave, which travels directly from the source to receivers, we can get the velocity of the rocks.Then by looking at the later arriving reflected and refracted scattered waves, we can see possibly where the structure changes and where the layering beneath the Earth is.